This invention relates to gas insulated switching apparatus and, more particularly, to a hybrid type gas insulated switching apparatus in which air insulation and gas insulation, for instance by SF.sub.6, are used in combination.
FIG. 1 shows an elevational view, partly broken away, showing a hybrid type gas insulated switching apparatus which has been proposed earlier by the inventors. This apparatus is constructed for applications with a very high voltage rating of, for instance, 550,000 volts. In this case, the air insulation distance L between a pair of bushings 2a and 2b in air is required to be about 6 m. The bushing 2a is supported upright on a support 4a. The support 4a also supports one end of a horizontal cylindrical tank 5a. The other end of the cylindrical tank 5a is supported on a different support 4b. Insulating gas, for instance SF.sub.6, is sealed together with a circuit breaker body 5b in the cylindrical tank 5a. One terminal section 5c extending from one end of the circuit breaker body 5b is supported by an insulating spacer 6 and is connected to one end of a dummy conductor 7. The dummy conductor 7 is sealed inside a casing 8, and an annular current transformer 9 is mounted on the outer periphery of the casing 8 concentrically with the dummy conductor 7. The casing 8 is supported on a support 12 which extends upright from a support base 11 provided on a foundation 10.
The other end portion of the dummy conductor 7 is supported by an insulating spacer 13 and extends into a disconnecting switch 14 where it is connected to one terminal section 14a of the switch 14. The lower end of the terminal section 14a is coupled to a switch mechanism section 15 of the disconnecting switch 14 mounted on the support base 11. When the switch mechanism section 15 is driven, a movable contact of the terminal section 14a is upwardly moved into contact with a fixed contact of the other terminal section 14b. As a result, a center conductor extending in the bushing 2b in air and the dummy conductor 7 are connected to each other via the disconnecting switch 14. On one side of the terminal section 14b, a fixed terminal 16a of a grounding switch 16 is disposed, and a movable terminal of the grounding switch 16 is provided on a casing 14c of the disconnecting switch 14 such that it faces the fixed terminal 16a. Although not shown, the movable terminal side of the switch 16 is grounded.
A conductor 17 projects from a different side of the terminal section 14b and penetrates two insulating spacers 18 and 19 to be led into the interior of a casing of a lightning arrester 20 and connected to a terminal section 20a thereof. An insulated cylindrical intermediate electrode 21 coaxially surrounds the conductor 17 between the insulating spacers 18 and 19. Designated at 22a and 22b are shields in air provided for enhancing the insulation of the bushings 2a and 2b in air, and at 23 a CT.
With the gas insulated switching apparatus of the above construction, the bushing 2b in air extends upright above the support base 11 and is supported at the lower end by the casing 14c of the disconnecting switch 14, that is, the casing 14c also serves as a support of the bushing 2b. The grounding switch 16 is provided for the purpose of draining the induced current in the power transmission line connected to the terminal section 14b to ground at the time of the maintenance and inspection. When the grounding switch 16 is operated, a great deal of metal particles are generated from the terminal sections and other parts being operated. These metal particles fall through the casing 14c and are accumulated on the insulating spacer 13 supporting the terminal section 14a of the disconnecting switch 14. Therefore, the insulating property of the insulating spacer 13 is deteriorated in long use. By way of example, in a high capacity power transmission system, for instance of 550,000 V and 12,000 A, the induced voltage and current are as high as 90,000 V and 1,500 A. To drain such high induced current, the grounding switch has to be provided with a contact section having a gas puffing function. Therefore, the switch mechanism section is increased in size to increase the size of the overall switching apparatus. To cope with the reduction of the dielectric breakdown due to metal particles, it is necessary to use a large size insulating spacer of the contamination resisting type, and this also promotes the size increase. Further, with the construction shown in FIG. 1 the grounding switch for grounding the terminal section 14b is disposed at a comparatively high level from the ground surface. Therefore, an operating floor is necessary for its operation and inspection. This increases the installation cost and also inconvenience is felt in operation and maintenance.
In a further aspect, while a breakdown voltage test is made when the gas insulated switching apparatus of FIG. 1 is installed, at this time a high test voltage is applied to the arrester 20 as well. Where a zinc oxide is used as an element in the arrester 20, it is deteriorated by the application of the overvoltage as the test voltage, and this is inconvenient.